1. Field of the Invention
The present invention relates to a device for controlling the air-fuel (A/F) ratio of an internal combustion engine, more particularly, to an air-fuel ratio controller which stops control of air-fuel ratio when an abnormal condition occurs in the heater which heats an oxygen sensor detecting oxygen concentration of exhaust gas and an oxygen pump in order to maintain activity of these.
2. Description of the Prior Art
According to the art proposed by Japanese Utility Model Application Laid-Open No. 62-18659 (1987), using a wide-range air-fuel ratio sensor composed of an oxygen sensor generating electromotive force responsive to the difference of oxygen concentration between atmosphere and exhaust gas from an internal combustion engine and an oxygen pump allowing pump current flow for taking oxygen into and out of exhaust gas for comparison with atmosphere, pump current is controlled in order that voltage outputted from the oxygen sensor can be a predetermined value. The proposed air-fuel ratio sensor detects the air-fuel ratio of the engine according to the magnitude of pump current. The air-fuel ratio sensor proposed by the above art can continuously measure the air-fuel ratio from rich to lean degree. FIG. 1 is a schematic block diagram of the heater circuit of the wide-range air-fuel ratio sensor cited above. Heater 87 heats the wide-range A/F ratio sensor 80 so that it can be held at the predetermined temperature, and thus, the A/F ratio sensor 80 is activated and correctly functions itself.
Nevertheless, the A/F ratio sensor 80 proposed by the above art has a simplified constitution in which the heater 87 is connected between battery 37 and the ground. As a result, it cannot detect failure in the supply of voltage to the heater 87 caused by insufficient contact of connectors for example. If the heater 87 malfunctions as a result of disconnection, a signal outputted from the A/F ratio sensor 80 deviates to the lean side.
If the fuel control system executes fuel feedback control operation in reliance on the value delivered from a faulty sensor, the actual air-fuel ratio deviates to the richer side than the objective value. This in turn lowers the driving characteristic and generates an inadequately combusted exhaust gas which may cause fire or trouble to occur in the engine.